This invention relates to a process of providing an adherent coating of polytetrafluoroethylene (PTFE) on the walls of a metal body, particularly the inner walls of a hollow metal body. The PTFE coating obtained enables the metal parts to resist corrosion and has excellent powers of adhesion to metal walls under vacuum, even at elevated temperatures.
PTFE is known for its resistance to chemical reagents, even at elevated temperatures, which makes it useful as a protective coating for metallic materials, such as pipes, valve bodies, containers, and the like, which come into contact with corrosive products. In these applications, the PTFE, used as a relatively thin coating, provides chemical resistance, while the mechanical strength is provided by the metallic material.
Starting with aqueous dispersions of PTFE or organosols it is possible to apply coatings to the inner walls of a hollow metal body by spraying. Spraying is usually followed by drying and heating. With a process of this kind, it is only possible to obtain thin, more or less porous coatings which cannot give the metal support effective long-term protection against corrosion by every chemical reagent.
To provide better protection, casings of PTFE are generally used, which are obtained previously be using conventional techniques such as extrusion or rolling in a strip, for example. These casings are inserted, under some pressure, as described in French Pat. No. 1,198,620, into the hollow metal body, which is thus protected against corrosion.
This technique has a major drawback, which is the poor behavior in a vacuum, or even at slight underpressure, of the PTFE casing, owing to the fact that it does not adhere to the metal walls.